The present invention relates to white illumination, particularly to white illumination of a display.
The human eye perceives three main components of light: blue, red and green. On the basis of the ratio of the intensities of the main components, vision produces the visual sensation experienced by the viewer. Since vision is based on the ratio of the intensities, it compares the ratios of the main colour components of the light reflected by the object displayed with the colour of ambient light.
The colours of images shown on a display should look natural, i.e. the ratio of the intensities of the main components should correspond to the ratio the object would reflect when displayed in natural light. This naturally requires that when the object is being imaged, the balance between the main components, i.e. the white balance, should be adjusted to natural. Correspondingly, when images are to be reproduced, the display should naturally be capable of producing the displayed image correctly, i.e. the ratio of intensities of the main components should be correct.
When images are to be displayed, light may be directed towards the viewer to produce the image. This takes place for instance in television where a coloured picture is formed using three colour projectors. Another method is to form the picture on the surface whereby ambient light or light passing through the surface makes the image visible on portions of the surface to the viewer. One example of this is liquid crystal displays, or LCDs. A drawback of these displays is that in dim light the image (or text) displayed is difficult to distinguish. This is why liquid crystal displays are often provided with back or front illumination to illuminate the display by allowing white light either to reflect from the portion of the liquid crystal display that forms the image or to conduct through it. However, the illumination should not distort the colours of the picture so that they look unnatural and this is why the illumination must be provided using white light.
There are various ways of producing white light, but in mobile stations, for instance, usually the appropriate way to illuminate the display is to use relatively small components of low energy consumption. U.S. Pat. No. 5,998,925 discloses one example of illuminating the display with white LED (Light Emitting Diode) components. The white LED in question is produced by coating a blue LED with broadband, yellow phosphor which renders the light emitted by the LED white. The phosphorous layer absorbs some of the intensity produced by the blue LED and corrects the colour of the light so that it becomes white, transforming, however, at the same time some of the luminous power into heat. In fact, a better efficiency can be obtained by using LEDs of the main component colours together to produce white light, as disclosed in U.S. Pat. No. 5,731,794. However, white light is difficult to produce with LEDs of different colours, because ageing and the temperature of the LEDs change their brightness.
In addition to the above problems, the white colour of the display varies not only because of a specific display illumination but also due to the impact of ambient light. Indoors, at the light of an incandescent lamp, the LCD display of a prior art laptop computer, which is illuminated by back or front illumination, looks typically slightly bluish, whereas outdoors it looks slightly reddish or yellowish. The phenomenon is caused by the change in the spectral distribution of ambient light.
The present invention provides a method and apparatus which allow the above problems to be avoided or their impact to be reduced.
According to a first aspect of the invention, there is provided a method for producing white light for illuminating an object, in which method light of a first colour having a first luminous power and light of a second colour having a second luminous power are produced to illuminate the object, the method being characterised in that it comprises the steps of
measuring the first luminous power;
measuring the second luminous power; and
adjusting the white balance of the object""s illumination by adjusting at least the first or the second luminous power at least on the basis of the luminous powers of the lights of the at least first and second colours.
Luminous power in this context refers to the light energy emitted by a light source per unit of time. The measurement of luminous power refers to the measurement of luminous power, or intensity, falling on a specific surface area, the measurement taking place at a specific distance from the light source and correlating with the emitted light efficiency.
An advantage of the method is that it provides an illumination having a white balance that can be corrected in response to changes in the illumination due to wearing of the light source or temperature changes, for example. In addition, the method allows lights of a plural number of different colours to be used for illuminating the object, while enabling a good white balance of the illumination to be maintained at the same time. The efficiency of the illumination is therefore better than for example if a light source of a single colour were used in which case some of the spectrum would be absorbed using a colour filter.
Preferably, lights of at least three different colours are produced for illuminating the object. Preferably the colours comprise blue, red and green. This allows an illumination approximating natural light to be provided.
Preferably the method also includes the measurement of the white balance of the object""s ambient light and the adjustment of the luminous power of the light to be produced that comprises at least one colour in relation to at least one of the other luminous efficiencies to make the luminous power correspond better to the measured white balance of ambient light. This allows to compensate for the impact of changes in ambient light on the colours of the object.
Preferably in the method, said luminous powers of lights of different colours are preferably measured alternately, using at least partially common processing means. The use of common processing means renders the method of the invention inexpensive and relatively easy to implement for use in current and future digital devices.
The white balance of the object""s illumination is preferably adjusted during illumination, for example, at periodically recurring intervals, or continuously.
Preferably the white balance of the object""s illumination is controlled at a specific slowness to stabilise the control. The control is preferably carried out in small steps.
Preferably a first measurement unit is applied for measuring the luminous power of light of a first colour in a significantly normalised illumination with regard to the light in question to allow at least one individual measurement unit characteristic to be determined and to be used for correcting a later measurement. This allows the error caused into the white balance adjustment by the difference in the measurements of ambient light and illumination to be reduced.
The object to be illuminated with the method is preferably a display, the method also involving the presentation of visual information on the display.
The intensities referred to are preferably measured using measurement devices arranged in the vicinity of the display.
According to a second aspect of the invention, an apparatus for illuminating an object with white light is provided, the apparatus comprising:
first illuminating means for producing light of a first colour having a first luminous power;
second illuminating means for producing a light of a second colour having a second luminous power;
the apparatus being characterised in that it further comprises
means for measuring the first luminous power;
means for measuring the second luminous power; and
means for adjusting the white balance of the object""s illumination by adjusting at least the first or second luminous power on the basis of the at least first and second luminous powers.
The apparatus preferably comprises more illuminating means for producing at least one light of a different colour. Yet more preferably the illumination of the object is carried out using lights of at least three different colours, which comprise blue, red and green light. This allows an illumination approximating natural light to be provided.
The apparatus preferably comprises also means for measuring the white balance of the object""s ambient light and for adjusting at least one luminous power in relation to the measured white balance of ambient light. This allows to compensate for the impact of changes in ambient light on the colours of the object.
The apparatus preferably also comprises at least partially common processing means for alternately measuring the different luminous powers. Yet more preferably the at least partially common processing means are also arranged to adjust at least one luminous power. Due to the at least partially common processing means the apparatus of the invention is inexpensive and fairly easy to implement.
The apparatus is preferably arranged to adjust the white balance of the object""s illumination during the illumination, for example, at periodically recurring intervals or continuously.
The apparatus is preferably arranged to adjust the white balance of the object""s illumination at a predetermined slowness to stabilise the control, and even more preferably in small steps.
The apparatus is preferably arranged to apply a first measurement unit to measure the luminous power of light of a first colour in a significantly normalised illumination with regard to the light in question to determine at least one individual measurement unit characteristic and to use it for correcting a later measurement.
Preferably the apparatus comprises a display for visual presentation of information, the display being the object to be illuminated.
The means for measuring the first luminous power and the means for measuring the second luminous power are preferably arranged in the vicinity of the display.
The apparatus preferably comprises a photoconductor arranged in the vicinity of the display for conducting light to the display. The means for measuring the first luminous power and the means for measuring the second luminous power are preferably arranged in the vicinity of the photoconductor to measure the intensities of illumination provided by lights of different colours conducted through the photoconductor.
According to an alternative embodiment a portion of the display is arranged to serve at least occasionally as means for measuring at least one luminous power.
The at least one set of illuminating means is preferably arranged to make light scintillate according to a PWM-signal (Pulse Width Modulation). The luminous power of these illuminating means is preferably adjusted by changing the cycle ratio of the PWM-signal to adjust the glimmering so that a desired luminous power will be produced.
The invention is suitable for use in diverse objects, such as mobile stations, electronic books, digital cameras, digital video cameras, portable computers, displays provided with back illumination and those provided with front illumination.